SUMMARY Duchenne muscular dystrophy (DMD) is a progressive muscle disease with severe cardiac complications. Dilated cardiomyopathy is a serious condition apparent in 100% of DMD patients over 18 years of age. Over 40% (and increasing) of DMD patients are dying from heart failure. However, studies of dystrophic cardiomyopathy remain quite limited. The culprit ? loss of dystrophin ? is a subsarcolemmal protein that protects skeletal and cardiac muscle from mechanically induced damage and regulates a number of downstream cellular pathways. While recent strategies to restore dystrophin levels keep hopes up, the medical and scientific communities are also seeking alternative therapeutic targets downstream of dystrophin. We found key evidence suggesting a detrimental role for connexin-43 (Cx43) in DMD-cardiomyopathy. We showed that Cx43 is upregulated and remodeled in the heart of mdxand mdx:utrophin knockout mice, as well as in the heart of DMD patients. We also showed that selective pharmacological inhibition of Cx43 function or genetic reduction of Cx43 leads to a rescue of arrhythmias and prevention of premature death in stressed mdx mice, as well as protection from the development of long-term cardiomyopathy. We determined that Cx43 in mdx is hypophosphorylated in a specific triplet of serine residues (S325/328/330). Reduction of phosphorylation in this same triplet promotes Cx43 remodeling in a variety of cardiac pathologies, leading to undesired opening of Cx43 hemichannels, arrhythmic susceptibility and irreversible long-term consequences. We believe that cardiac remodeling of Cx43 in DMD contributes to cellular Ca2+ overload and oxidative stress, leading to arrhythmias, fibrosis and consequently, to the development of heart failure. We believe that the phospho-triplet S325/328/330 is the driving force for Cx43 remodeling. In this proposal we will first determine whether expression of a mutant form of Cx43 protects mdx mice from developing cardiac pathology on functional and histological (Aim 1) as well as on cellular (Aim 2) levels. To address this question, Cx43 knockin mice will be crossed with mdx mice, to incorporate the Cx43 phospho-mutation harboring glutamic acids instead of serines in the phospho-triplet thus mimicking permanent phosphorylation. In Aim 3 we will investigate molecular mechanisms responsible for enhanced opening of Cx43 hemichannels in dystrophic hearts. In particular, we will examine if excessive ROS opens the hemichannels. Rigorous blinded functional, histological and biochemical analysis will be conducted.